In this invention, the amorphous silicas are differentiated by structure into three classifications; namely, TYPES I, II and III.
Type I silica, defined here, is precipitated from geothermal water. This silica is commonly sourced for commercial use by acid treatment of sodium silicate solutions prepared for example, by the digestion of quartz sand with sodium hydroxide.
Type II and III silicas, defined here, may be precipitated from geothermal water or an alternate aqueous source. This alternative feedstock fluid should contain sufficient initial silicic acid and or silicate ion to allow controlled precipitation as described for geothermal water in this patent.
Type II precipitated silica is similar in structure to a class of commercial silicas normally prepared under anhydrous conditions, known as pyrogenic "arc" silicas. The "arc" process involves the oxidation of silicon monoxide [SiO] at very high temperatures under anhydrous conditions.
Geothermal water is normally characterized as being slightly acidic to neutral to slightly alkaline with significant concentrations of SiO.sub.2, Na, K, Li, Ca, Mg, Rb, Cs, B, SO.sub.4, Cl and dissolved gaseous species, e.g. CO.sub.2, H.sub.2 S and NH.sub.3. Almost always, arsenic is present. A detailed chemical composition is shown in Table 8.
Arsenic contamination of precipitated silicas from a geothermal source may or may not be a problem, depending on the end usage of the precipitated silica. Thus while in some applications procedures must be adopted whereby the arsenic contamination of silica is reduced, for others it may not be necessary. For example, where amorphous silica is being precipitated for paper coating or impregnation usage, preferably the arsenic concentration is low, e.g. less than 2 mg kg.sup.-1 in the paper sheet.
The present invention is directed to controlled processes which give rise to precipitated amorphous silicas having a structure which is different from that which occurs naturally upon cooling and deposition of silica from geothermal waters. Unlike such naturally occurring amorphous silicas, the precipitated amorphous silicas of the present invention have what is termed a "tertiary aggregate structure". A "tertiary aggregate structure" is a three dimensional structure consisting of aggregates of primary particles into secondary particles (substantially spherical) which themselves may be bridged by other accumulations of the primary particles (elongate accumulations) into the tertiary aggregate structure. Bridging silica is more frequently contained in Type I silica.
The precipitated silicas described in this invention are classified according to the structure they possess and are identified as: